A former MI5 agent has told Sky News that the agency is foiling terror "all the time" and sometimes daily.

'Tom Marcus' spent 10 years as a surveillance agent for British intelligence. Tom isn't his real name, and my interview with him was authorised by MI5.

"If we'd been briefed on a target that we needed information from, we'd go and hunt them," he said.

"We'd go and find everything we can about them, we'd watch everything they do. Essentially, as soon they come onto our grid and we need to know all information about them, our surveillance teams, myself included, would go out there and find them and make sure we control everything they do."

Video:Ex-MI5 man on capture of terror suspect

On one operation, Tom was nearly kidnapped by the terrorists he was tracking.

"(This) was something we'd never, ever seen before on UK soil, and it came very, very close to them putting me into a van and taking me to an address that we later searched with Special Branch, where they found plastic sheeting on the ground and a video camera and the flag and the butcher's knife. It was really down to the wire."

On another occasion, Tom was part of an MI5 team tracking a suspected Islamist terrorist. He sat outside a London mosque during evening prayers.

He said: "It was chucking it down and I was dressed as a tramp, pretending to ask for change, my own clothes soaked in my own urine to compliment that cover.

"There were more woman worshippers that had left the mosque than had gone in, so I alerted the team that there was a possibility he had changed his appearance."

Image:'Tom' worked for MI5 as a surveillance agent for 10 years

The suspect had changed into a burka and was pretending to be a woman. Following him, the MI5 team called in a Special Forces unit to arrest the terrorist. Two Range Rovers slammed into the suspect's car.

"As the team came in incredibly quick in the Range Rovers (and) dragged him out of the car, they found weapons and pipe bombs in the boot and his planned target was the very next day because this was all at night.

"To be waiting for a couple of coach loads of children returning from a trip to France and he was going to kill them, the teachers and the parents."

It is not just Islamic extremism that threatens UK security. Chinese and Russian agents operate on British streets and they are good at what they do.

"The Russians, particularly the Russians, their intelligence agents are extremely effective," he said.

"They are so well trained in counter-surveillance; the ability to stop and identify operators like me."

Tom left MI5 with post-traumatic stress. With the agency's permission, he has written a book about his time as a spy.

As we walk and talk just across the road from MI5 headquarters he tells me how colleagues of his were killed on operations, but he says he never did it for recognition, and he misses the job every day.